New high strength stainless steel alloys like Reynolds 953 make possible thinner walled (lighter) steel frames that compare favorably with titanium and carbon fiber composite frames in terms of weight without compromising strength or durability, and without the low elongation (brittleness) of carbon fiber.
Stainless steel tube sets have been available for some time (Columbus offered one called Metax, IIRC, for a while) and the new Reynolds 953 is stronger than most. However the tubes are very expensive and difficult to machine and form. They also require similar inert-gas welding techniques to Al and Ti so the fabrication and joining processes are at least as costly as Al or Ti. They have never caught on.
Al does have it's mechanical limitations but they are more academic than real in the commercial world. Broken bike frames do happen but sudden, non-accident induced failures are very rare. Al is cheap, light, easy to fabricate and the welding techniques are well known. It's fatigue life is finite but plenty long.
As an aside, do you use steel for all of your bike parts? Handlebars, stems, cranks, etc. are almost exclusively Al (or carbon) these days and your safety is actually more dependent on these items than the frame in case of sudden breakage.